1. Field of the Invention
This invention relates to the field of automotive power steering systems and, more particularly, pertains to a system for driving a power steering pump at a variable speed concurrently from the engine and from the transmission output.
2. Description of the Prior Art
An automotive power steering system usually includes a hydraulic pump and a control valve connected to the discharge side of the pump, which directs pressurized hydraulic fluid at a specific flow rate to the right or left-hand portions of the power steering gear depending upon the direction that the steering wheel of the vehicle is turned by the operator. Several kinds of hydraulic pumps are used in such systems including sliding vane, slipper or gerotor types; these usually are belt-driven from the engine shaft and adapted to produce a volumetric flow rate that varies in proportion to the speed of the engine. The flow capacity of the pump is determined with the engine idling, the wheels of the vehicle stopped and the steering wheel being turned at a fast rate of speed. The flow capacity of the pump established during these conditions is the capacity sufficient to keep the power steering system pressurized and simulates the frequently occurring parking maneuver operating condition. However, at highway speed, the pump produces a flow rate substantially greater than that required in the idle speed condition and may, at the higher engine speeds, produce a flow rate that is several times greater than the idle speed flow rate. This excess flow capacity is returned to a hydraulic fluid sump which is open to the inlet side of the pump for recirculation. There is, therefore, associated with this conventional power steering system a considerable loss of efficiency when the engine is operating at speeds greater than the idle speed. It is preferable that a power steering system actually produce less flow as the engine speed increases because as a practical matter at highway speeds, the steering wheel of the vehicle can be turned safely through only a small angle, otherwise the vehicle is subjected to the possibility of roll-over.